1. Field of the Invention
This invention relates to a device for demolding a plastic injection molded part with an undercut, particularly a device that eliminates burrs and improves mold release.
2. Discussion of Related Art
Some known injection molding systems include a mold with a moveable part and a fixed part. A molded part or piece is formed between the moveable part and the fixed part, within a mold, for example when the mold system is in a closed position.
As the mold system moves from the closed position to an extended position where the moveable part is at a distance from the fixed part, the molded part can be ejected or otherwise removed by known ejector pins and/or another conventional ejection device.
The designs of conventional ejection devices become more complex when the molded part forms an undercut or another similar negative, void, space or volume. There is a need for an ejection device that can operate efficiently when demolding a molded part, particularly a part with an undercut or a negative, void or space.
FIGS. 5 and 6 show a conventional mold for molding plastic pieces including a fixed part 40 and a movable part 41. A die plate is mounted to the fixed part 40 and another die plate is fixed to the moveable part 41. These two plates form the part or piece 42 to be molded. The moveable part 41 of the mold is closed against the fixed part 40 and forms a molding chamber between the two die plates.
The moveable part 41 of the mold is later separated from the fixed part 40, and the finished or molded piece 42 remains affixed to one of them, normally to the plate corresponding to the moveable part 41 of the mold. The piece 42 is removed using an ejection device or another suitable removal apparatus.
The molded pieces 42 are usually ejected using pins or sterns which push the molded piece 42 out of the die plate. Pins or stems can emanate from an ejection plate 51 which is incorporated so that it floats about the moveable part 41 of the mold. When the ejection plate 51 moves then the pins move the molded piece 42 to be demolded.
In the plastic molding industry, the areas of the mold which do not have the same demolding direction as the remainder of the pieces are known as undercuts, negatives and/or voids. For example, a gripper 45 for the piece 42 cannot be demolded in the direction used to directly extract the piece 42, because it is buried in the piece 42.
For demolding, some movable counter-molds 4 move forward with the ejection plate 51 and also open on the sides. This opening or lateral movement releases the negative and allows the piece 42 to be ejected, extracted or removed, such as shown in FIGS. 5 and 6.
FIG. 5 shows a one embodiment of the counter-mold 4. The counter-mold 4 includes a rectangular or circular bar 47, which is laterally lowered to allow a lateral flexion and release the negative from the mold.
Another embodiment of the counter-mold 4 is shown in FIG. 6. The counter-mold 4 includes a cylindrical runner 6 and a collapsible head 7 articulated together. The articulation 50 allows the collapsible head 7 to move laterally and to release the negative.
One problem with the first prior art embodiment can occur if replacement is required due to breakage or wearing, because the entire element must be replaced which requires complete disassembly of the mold support plates as well as considerable time to machine the housing of the piece, which relates to costs.
The second prior art embodiment can have no problems but the adjustment for the two pieces forming the counter-mold is cylindrical with reduced conicity because the injection pressure for the plastic can change the position of the collapsible head 7, and thus cause a change in the geometry of the pieces, burrs, and the like. As the cylindrical part has a greater surface it needs to travel further to collapse the first piece.
Some of these problems are solved by an invention taught by Spanish Patent Application 200502882 which refers to a negative demolding system, and covers the layout of a spring between the runner and the head to improve its collapsibility, and the layout of a tilted lateral face in the collapsible head which relocates the head as it moves back into the bushing.
The invention of Spanish Patent Application 200502882 helps meet some objectives but this invention moves forward to meet other objectives with the development of an advantageous demolding system.